Historically, automatic pilots for aircraft, such as described by Lewis et al in U.S. Pat. No. 2,853,671, and Moseley in U.S. Pat. No. 2,126,910, drove an aircraft control surface by means of a motor in response to an attitude signal or a command signal. The motor drives a control surface and a follow-up element serves to rebalance the system. Such systems generally have been characterized by the use of a follow-up element, either mechanical or electrical, which is driven along with the control element. Without such follow-up control an aircraft has often been unstable, particularly in aileron control systems. In such systems, the aircraft oscillates banking back and forth, angles up and down about the pitch axis, the angles dependent upon the particular system and aircraft. The purpose of the follow-up has been to provide damping of such oscillations. This has been accomplished by making the control system deflection proportional to the system error signal, where the system error signal represents the difference between a control or command signal and an attitude or altitude signal. A signal, either a mechanical motion or an electrical signal, proportional to the control surface deflection is fed to a summation point so as to produce a control surface deflection directly proportional to the system error signal. Such aircraft control systems have developed from functionally simple but electromechanically complex systems which were designed so that they could be adapted to a large variety of aircraft types of today's functionally, electronically and mechanically complex systems.
More recently, flight control systems have been developed which employ an open loop drive for the control surface, such as described by Younkin in U.S. Pat. No. 3,686,555. An open loop is provided in which the controlled element is made to follow an input signal without any direct comparison between control element movements and the input signal. This is accomplished by employing a control amplifier which matches the characteristics of the controlled element to the input signal.
In accordance with the present invention, an open loop control is provided wherein a rate signal is generated and compared with a motor velocity signal to generate a motor drive voltage for driving the servomotor to produce a control surface activation rate proportional to the rate signal and the motor velocity signal. A rate circuit responds to an attitude gyro output to generate the rate signal to be combined with the motor velocity signal. The motor velocity signal is provided by a detector responding to motor operation. Thus, the automatic flight control system of the present invention provides an open loop control with improved response.